Interpretive Summary: This is an invited commentary on a paper of ours that finds that catechin is not involved in allelopathy of the invasive plant spotted knapweed. Additional data are provided showing that catechin is rapidly converted to colored products by extracellular enzymes of roots.

Technical Abstract:
Proving allelopathic chemical interference is a daunting endeavor, in that production and movement of a phytotoxin from a donor plant to a receiving plant must be demonstrated in the substrate in which the plants grow, which is usually a complex soil matrix. The soil levels or soil flux levels of the compound generated by the donor must be proven to be sufficient to adversely affect the receiving plant. Reports of (–)-catechin to be the novel weapon used by Centaurea stoebe (spotted knapweed) to invade new territories are not supported by the paper featured in this addendum, nor by papers produced by two other laboratories. These papers find that (–)-catechin levels in soil in which C. stoebe grows are orders of magnitude below levels that cause only minor growth effects on reported sensitive species. Furthermore, the claim that (–)-catechin acts as a phytotoxin through causing oxidative damage is refuted by the fact that the molecule is a strong antioxidant and is quickly degraded by extracellular root enzymes.